This invention pertains to a process for the preparation of trifluoromethanesulfonic acid anhydride (triflic anhydride) by the disproportionation of a mixed anhydride comprising a trifluoromethane-sulfonyl acyl residue and a carboxyl residue. More particularly, this invention pertains to a process for the preparation of trifluoromethane-sulfonic acid anhydride by the steps of (1) forming a mixed anhydride comprising a trifluoromethanesulfonyl acyl residue and a carboxyl residue by contacting trifluoromethanesulfonic acid or a derivative thereof with a carboxyl compound selected from ketene, dialkyl ketenes, carboxylic acids, and derivatives of carboxylic acids and (2) subjecting the mixed anhydride to reactive distillation wherein the mixed anhydride undergoes disproportionation to produce triflic anhydride and a higher boiling carboxylic acid anhydride.
The preparation of trifluoromethanesulfonic (triflic) acid was first reported in 1954 by R. N. Hazeldine and J. M. Kidd, J. Chem. Soc. 1954, 4228. It is widely used in organic synthesis both for research and commercial interests. See, for example, F. J. Waller, D. Ramprasad, A. G. M. Barrett, D. C. Braddock, Chem. Ind., 1998, 75, 289-305; Kurt Ritter, Synthesis, 1993, 735-62; P. J. Stang, M. Hanack and L. R. Subramanian, Synthesis, 1982, 85; and R. D. Howells and J. D. McCown Chem. Rev., 1977, 77, 69-92. Synthesis of triflic anhydride was first reported by Brice and Trott in U.S. Pat. No. 2,732,398 (1956) and in the following year higher yield preparations via treatment of trifluoromethanesulfonic acid with phosphorus pentoxide were reported by Burdon et al, J. Chem. Soc., 1957, 2574 and Gramstad et al., J. Chem. Soc., 1957, 4069. 
In the ensuing four decades this phosphorus pentoxide approach has remained the preferred synthetic method. Japanese Published Patent Documents JP 11-236,365 and JP 11-236,366 describe the preparation of triflic anhydride by the reaction of triflic acid with phosphorus pentachloride or phosphorus trichloride and chlorine. Such reagents are inefficient as they generate a volume of non-recyclable byproducts as large as, or larger than, that of the desired product. These reagents, therefore, are not well suited for large-volume manufacture of triflic anhydride.
We have discovered an improved process for the preparation of triflic acid anhydride which does not require the use of a phosphorus reagent. The present invention provides a process for the coproduction of trifluoro-methanesulfonic acid anhydride and a carboxylic acid anhydride which comprises the steps of:
(1) forming a mixed anhydride comprising a trifluoromethanesulfonyl acyl residue and a carboxyl residue by contacting trifluoromethanesulfonic acid or an acid halide thereof, e.g., trifluoromethanesulfonyl chloride, with a carboxyl compound selected from ketene, dialkyl ketenes, carboxylic acids, salts of carboxylic acids, and carboxylic acid halides; and
(2) subjecting the mixed anhydride to reactive distillation to produce triflic anhydride and a higher boiling carboxylic acid anhydride.
In an especially preferred embodiment of the present invention, triflic acid is reacted with dimethylketene to produce the mixed anhydride isobutyryl triflate (or trifluoromethanesulfonyl isobutyrate), i.e., a mixed anhydride having the formula:
F3CSO2xe2x80x94Oxe2x80x94C(O)xe2x80x94CH(CH3)2 
which then is disproportionated by means of reactive distillation to produce the lower boiling triflic acid anhydride and the higher boiling isobutyric anhydride. Our novel process provides a number of advantages such as producing triflic anhydride in high yields, being readily adaptable to continuous operation and producing essentially no waste products.